I would attempt to remove the rust without actually removing and material(sanding, blasting, ect.) this will typically give you a good idea of the extent of the rust. There are plenty of industrial rust removers that can be purchased. My favorite method is as follows:
Electrolytic rust removal,
Mix 1 part baking soda or the prefered washing soda with 10 parts water. Next take a car battery or car battery charger and hook the positive lead to the part to be cleaned and submerge it into the soda/water solution. Next slowly drop the negative lead into the solution. Let it sit for about 10 minutes. The parts should bubble, and the rust will simply wash away.
This method is pretty simple and will yeild great results...just make sure you connect the leads in the proper order just the same as jumping a car.
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